Examples of hydraulically-actuated fuel injection systems are shown in U.S. Pat No. 5,191,867 issued to Glassey, et al. on Mar. 9, 1993, and U.S. Pat. No. 5,213,083 issued to Glassey on May 25, 1993, both being assigned to the assignee of the present invention. Engines equipped with a hydraulically-actuated fuel injection system (HEUI fuel system) employ an actuating pump to provide actuating fluid at elevated pressures to injectors, intensifying the pressure of the fuel being injected into the engine. Control of the fuel injection pressure is achieved by controlling the pressure of the actuating fluid. Typically, control of the actuating fluid pressure is achieved by employing a fixed displacement pump to elevate the fluid pressure and regulating that pressure to lower levels by bleeding off unneeded flow volume through a rail pressure control valve, past which the unneeded fluid returns to an actuating fluid sump such as an engine oil pan. While this is an acceptable and cost effective approach for many HEUI fuel system applications, it would be desirable in other applications to better match the displacement of the pump to the system flow requirements which vary over engine operating conditions and applications.
The present invention is directed to overcoming the problem as set forth above.